1. Field of the Invention
This invention relates to a data reproducing system for digital information signals superposed on television signals.
2. Description of the Prior Art
Recently, in the television broadcasting, digital signals, such as teletext signals, still image signals and facsimile signals, have been superposed on television signals so as to be transmitted, thereby having commenced various services. On the other hand, the signal processing unit for broadcasting equipment has made progress in digitization, integration, and size-reduction.
As disclosed in Japanese Laid-Open Patent Application No. 55066184, the conventional data reproducing system for the teletext signal comprises a waveform shaping circuit, a slicing circuit, a sampling circuit, a bit sync signal sampling circuit, a frequency multiplier circuit, a crystal oscillator exciter circuit, and a clock generator circuit.
The bit sync signal sampling circuit samples the bit sync signal (clock run-in signal) from the teletext signal. The frequency multiplier circuit doubles the bit sync signal so as to be a signal of 5.73 MHz which is equal to the transmission clock frequency of the teletext signal j, the signal of 5.73 MHz excites a crystal of 5.73 MHz in the crystal oscillator exciter circuit, and thereafter continues the excitation for one horizontal time period. The output of the exciter circuit is waveform-shaped in the clock generator circuit so as to be outputted as the sampling clock. Also, the television signal binary-coded by the waveform shaping circuit and slicing circuit is outputted in synchronism with the sampling clock to thereby reproduce data of the teletext signal.
The television signal, however, is subjected to group delay distortion by ghost images or repeaters during the transmission to receivers, whereby the teletext signal is distorted so as to greatly increase its error rate during the data reproduction in the data reproducing circuit.
To cope with the above, a waveform equalizing circuit is used to equalize the waveform of an input signal to the data reproducing circuit, which comprises a plurality of waveform equalizing circuits having different characteristics respectively, and which selects the waveform equalizing characteristic by switching the circuits corresponding to the distortion of the input signal.
With the above-mentioned construction, however, it is difficult to realize the optimum waveform equalizing characteristic with respect to the waveform distortion generated by a transmission line, because the waveform equalizing circuit are limited in number, and such a construction is difficult to automate because of the switching.
On the other hand, as disclosed in Japanese Laid-open patent application No. 56-16674, there is a waveform equalizing circuit using a transversal filter, which comprises a signal delay element, a signal multiplier, a signal adder, and a tap coefficient correction circuit. The transversal filter comprises a signal delay element, a signal multiplier, and a signal adder circuit. The tap coefficient correction circuit controls the transmission characteristic of the transversal filter so as to be of a reverse characteristic corresponding to the distortion in the input signal.
The conventional data reproducing circuit provided with the waveform equalizing circuit as above has the following problems:
(1) The sampling clock is generated only by the bit sync signal of 16 bits, whereby it is difficult to design and adjust a circuit for generating a stable sampling clock for one horizontal time period.
(2) The bit sync signal for deciding the sampling clock indicates the accurate sampling information after waveform-equalizing the input signal. When the waveform equalizing circuit is digitized, at first the signal must be sampled and quantized, but for the above reason, the accurate sampling timing is not obtainable before the waveform equalization.